The strikes hit three sites -- one in Damascus and two in Homs -- which US President Donald Trump said were "associated with the chemical weapon capabilities of Syrian dictator Bashar al-Assad."

The action followed a week of threats of retaliation for an alleged chemical weapons attack on civilians in Douma, outside Damascus, where Syrian forces have long been battling rebels.

Russian President Vladimir Putin called the missile strikes an "act of aggression against a sovereign state" and said they were against the United Nations' charter.

Russia -- a key ally of the Assad government -- is calling for an immediate UN Security Council meeting to discuss the "aggressive actions" of the United States and its allies, he said.

Latest developments

German Chancellor Angela Merkel said she supported the military action.

UK Prime Minister Theresa May said the military action was believed to have been successful.

French Minister for the Armed Forces Florence Parly said all French aircraft had returned safely from their mission.

Syria's Foreign Ministry condemned the airstrikes as a "flagrant violation of the international law."

Three civilians were wounded in Homs after "several" missiles were intercepted by Syrian air defense systems, Syrian state TV said.

Syria's Foreign Ministry called on the international community "to strongly condemn this aggression," warning it would "pose a threat to international peace and security as a whole," in a statement published by the state-run Syrian Arab News Agency.

Earlier, the office of the Syrian Presidency tweeted a video of Bashar al-Assad going to work Saturday, with the caption "a morning of steadfastness."

Danny Makki, a British-born Syrian journalist in Damascus, told CNN his whole neighborhood had been woken by the strikes near the capital. "You could tell straight away that this wasn't your average Damascus night-time battle. It was something far bigger," he said.

In a televised address announcing the action, Trump said he had decided to take action because last weekend's action by Assad "was a significant attack against his own people," and "not the actions of a man, they are crimes of a monster instead."

Trump said the purpose of the campaign was to "establish a strong deterrent against the production, spread and use of chemical weapons."

Gen. Joseph Dunford, chairman of the US Joint Chiefs of Staff, told reporters at a Pentagon briefing that the first allied strike targeted a scientific research center in greater Damascus involved in the development and production of chemical weapons.

The second site targeted was a chemical weapons storage facility west of Homs, while the third was a chemical equipment storage facility and important command post.

The strikes -- the strongest concerted action yet by Western forces in Syria -- were launched at 9 p.m. ET, as most of Europe and the Middle East was shrouded in darkness.

The Syrian Armed Forces said in a statement that 110 missiles were fired on Syrian targets and that the country's defense systems "intercepted most of the missiles, but some hit targets including the Research Center in Barzeh."

Three civilians were wounded in Homs after "several" missiles were intercepted by Syrian air defense systems, Syrian state TV said.

Russian state news agency TASS reported that none of the missiles fired by the three western nations struck areas near its naval and air bases in Syria. Those bases come under the protection of Russian air defense units.

A meeting of the North Atlantic Council will be held Saturday afternoon, a NATO official told CNN. France, the United Kingdom and the United States will brief allies on actions taken in Syria, the official said.

Theresa May: 'Not about regime change'

In a televised statement Saturday, UK Prime Minister Theresa May said UK forces had undertaken a "limited and targeted strike" and that there had been "no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime."

While a full assessment has not yet been completed, "we believe that the action was successful," she said. "This was not about interfering in a civil war. And it was not about regime change."

While insisting that the military action was a direct consequence of the alleged chemical weapons attack in Douma, May also alluded to the use of a nerve agent in the English city of Salisbury. "We cannot allow the use of chemical weapons to become normalized -- within Syria, on the streets of the UK, or anywhere else in our world," she said. Britain says Russia is behind the Salisbury poisoning, a charge Moscow denies.

Experts from the international chemical weapons watchdog, the Organization for the Prohibition of Chemical Weapons, were en route to the alleged chemical weapons attack site in Douma before the joint military strike was launched.

Russia's Ministry of Defense said it believed the action by the US, UK and France was not in retaliation to the alleged chemical attack in Douma but instead was a reaction to the "success" of the Syrian army in liberating its territory from rebels.

International reaction

Turkey, an important player in the Syrian conflict, said it viewed the airstrikes as "an appropriate response" to the Douma attack.

"We welcome this operation which has eased humanity's conscience in the face of the attack in Douma, largely suspected to have been carried out by the regime," Turkey's Foreign Ministry said in a statement.

Merkel said all available evidence pointed to the Assad regime being responsible for the Douma attack and criticized Russia for blocking an independent investigation through the UN Security Council.

"The military operation was necessary and appropriate to preserve the effectiveness of the international ban on the use of chemical weapons and to warn the Syrian regime of further violations," she said in a statement voicing support for the joint US, UK and French action.

Australia's defense minister issued a statement in support of the strikes, calling them "a calibrated, proportionate and targeted response."

An Israeli official told CNN that the strikes enforced the red line drawn by President Trump last year. "Syria continues to engage in and provide a base for murderous actions, including those of Iran, that put its territory, its forces and its leadership at risk," the official said.

However, Iran -- another key ally of the Assad government -- condemned the strikes. "The attack is the blatant violation of international laws, as well as ignoring the sovereignty and territorial integrity of Syria," Iranian Foreign Ministry spokesman Bahram Qasemi said according to Iranian state media.